An airplane's airframe and engines may produce varying amounts of audible noise and turbulent drag during different flight conditions. One of the main sources of noise and drag is the air flow around aerostructure surfaces. Leading and trailing wing surfaces, control surfaces, landing gear structures, air flow around turbofan engine surfaces, and turbofan engine exhaust flow may produce noise. As flight conditions change, the velocity, temperature, pressure, turbulence, and other properties of the air and exhaust can change considerably. On takeoff and landing, the external air (free stream air) velocity may be lower, temperatures higher, and engine exhaust power at a maximum (i.e., for takeoff). During cruise, the external air (free stream air) velocity may be higher, temperatures lower, and turbofan engine exhaust power at a cruise level. From ground to cruising altitude, all of these factors may vary in complex non-linear ways for various flight conditions.
In order to improve aircraft performance across all phases of flight such as by reducing takeoff noise and reducing drag during cruise while minimizing weight, an aircraft design should include optimized shapes and physical properties (such as stiffness) of the aerostructures. However, the optimal shape and other properties change depending on the flight conditions. Thus, it may be desirable for an aerostructure to be dynamically reconfigurable in order to change to adapt to the current flight conditions.
Of particular interest is the noise and drag from the engines. Conventional turbofan engines include a fan section and an engine core, with the fan section having a larger outer diameter than that of the engine core. The fan section and the engine core are disposed sequentially about a longitudinal axis and are enclosed in a nacelle. An annular path of primary airflow (core flow) passes through the fan section and the engine core (core nozzle) to generate primary thrust. An annular path of fan flow, disposed radially outward of the core airflow path, passes through the fan section and exits through a nozzle (fan nozzle) to generate fan thrust.
The requirements for takeoff and landing conditions are different from requirements for a cruise condition. For cruise conditions, it is desirable to have a smaller diameter fan nozzle for increasing cruise performance and for maximizing fuel efficiency, whereas, for takeoff and landing conditions, smaller diameter fan nozzles may not be considered optimum. Therefore, in many conventional engines, cruise performance and fuel efficiency are often compromised to ensure safety of the turbofan engine at take-off and landing. In addition to improved efficiency, varying the fan nozzle area and hence the engine bypass ratio is an extremely effective means of reducing community noise during takeoff and approach. Some turbofan engines have implemented variable area fan nozzles (VAFN). VAFN have the ability to have a smaller fan nozzle diameter during cruise conditions and a larger fan nozzle diameter during take-off and landing conditions.
With present day jet aircraft, structures typically known in the industry as “chevrons” have been used to help in suppressing noise generated by a jet engine. Chevrons have traditionally been, triangular, tab-like elements located along a trailing edge of fan and core nozzles of turbofan jet engines such that they project into the exhaust gas flow stream exiting from the fan and core nozzles. For a wide range of operating conditions, chevrons have proven to be effective in reducing broadband noise generated by the mixing of airflows from the core nozzle and fan nozzle, and the mixing of airflows from the fan nozzle and free stream air. Since the chevrons can interact directly with the fan flow, however, they also generate drag and loss of thrust. Consequently, there is a tradeoff between the need to attenuate noise, and minimizing the loss of thrust due to the presence of the chevrons.
Thus, there is a need for technology which provides the needed noise attenuation but does not produce additional drag or loss of thrust during cruise conditions.